The present invention relates to a method and apparatus for controlling the air-fuel ratio of a mixture supplied to an internal combustion engine in accordance with the operating conditions of the engine.
2. Description of the Prior Art
In the past, a method has been put to practical use in which a stoichiometric air-fuel ratio (the ratio of air and fuel supplied or A/F=about 15:1) is detected from the composition of the exhaust gases of an engine by an air-fuel ratio sensor positioned in the exhaust pipe of the engine and the air-fuel ratio to be supplied is controlled around the stoichiometric air-fuel ratio in accordance with the detection signal. This method provides a very effective method of purifying the exhaust gases if it is used in combination with a three-way catalyst.
From the standpoint of fuel consumption, however, generally it is advantageous to determine the air-fuel ratio greater than the stoichiometric ratio or use a mixture leaner than the stoichiometric mixture. The air-fuel ratio of this lean mixture that provides the optimum fuel consumption is referred to as the optimum air-fuel ratio. Methods have been devised for controlling the air-fuel ratio at around the optimum air-fuel ratio and this air-fuel ratio is very close to one which causes the engine to misfire thus giving rise to disadvantages that during the periods of acceleration and deceleration the air-fuel ratio is varied thereby causing the engine to misfire and increasing the breathing, deceleration shock or vibrations and so on.